TAP 406-2: Plotting equipotentials
Here you study the shapes of various fields. The fields are created (in two
dimensions) between conducting electrodes of various shapes on a piece of
conducting paper. It will be easy to see equipotentials and their relationship to
the field itself.
You will need:
power supply, 5 V dc with one or two 4 mm leads
multimeter with one probe
conducting paper
carbon paper
plain white paper
softboard, a little larger than A4
various shaped electrodes made from copper strips (see below)
eight drawing pins
two drawing pins soldered onto 4 mm leads
crocodile clips
The experiment
1)
Before you are able to make measurements, you will have to set up a
‘recording system’ which makes use of carbon paper below the
conducting paper. Place the softboard on the bench and put a sheet of
white paper on this. Cover this with carbon paper (carbon side down)
and, on top, the conducting paper. All three sheets have to be held
firmly in place with drawing pins at each corner.
electrode
conducting paper
carbon paper
white paper
soft board base
2)
The two copper electrodes need to be secured very firmly. The
diagram below shows the set-up for parallel electrodes to create a
uniform field. These should be placed close to and parallel to the
longer edge of the conducting paper and then secured by pushing
drawing pins through the small holes in each strip. The central hole in
each strip is for a drawing pin to which a 4 mm lead has been soldered,
allowing you to set up this circuit:
copper electrode
held by pins
5V
V
conducting paper
3)
When you have set up this circuit, touch the probe on the end of the
flying lead gently onto the conducting paper. The voltmeter records the
potential difference between the point where the probe touches the
paper and one copper electrode. You should be able to see the
potential difference vary as you move from one strip to the other. If you
keep the distance from each strip the same (i.e. move parallel to the
electrodes), you find that the voltmeter reading does not change. You
are following an equipotential.
4)
Check that the supply is set to 5 V by placing the probe on the ‘positive’
strip. Now move the probe until the voltmeter reads 1 V and then push
down firmly on the paper. This will make a mark on the white paper
below. Find other points where the potential is 1 V and mark their
position in a similar way. Five or six points will allow you to draw an
equipotential when the upper layers of paper are removed (but don’t
remove them yet!).
5)
Repeat the procedure described above for potentials of 2, 3 and 4 V
and then mark the position of each electrode by using probe to draw
round each strip. Remove the conducting paper and carbon paper, to
see a series of dots on the white paper. Join these to show the position
of the equipotentials.
6)
Draw one or two field lines between the two electrodes (remember that
these will be at right angles to the equipotentials, including the strips).
7)
Repeat the procedure for other shapes of field by using electrodes of
various shapes. A couple of examples are given below:
electrode,
conducting
paint
5V
V
conducting
paper
Field close to a sharp point
electrode,
conducting
paint
5V
V
conducting
paper
Radial Field
Practical advice
This is a simple introduction to field lines and equipotentials.
Simply using the end of a 4 mm plug to mark points onto the white paper
below the carbon paper and conducting paper is quick and easy. There are
many ways of measuring and marking the potential at a point with specially
constructed probes. These can be made from graphite pencils, used ball-point
pens with a lead soldered onto the metal tip, or old instrument probes. These
will give a finer mark, but adequate results can be obtained from 4 mm plugs.
The electrodes can be painted using conducting paint which gives better
contact but is expensive.
External reference
This activity is taken from Advancing Physics chapter 16, 60E, 220E, 240E